Noise reduction sound system and method



B. KREUZER NOISE REDUCTION SOUND SYSTEM AND METHOD Filed Feb. 8, 1945 April 26, 1949.

n wm @Mq um S www Patented Apr. 26, 1949 UNITED NOISE REDUCTION SOUND SYSTEM AND METHOD Barton Kreuzer, Philadelphia, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application February 8, 1945, Serial No. 576,811.

7 Claims. 1

This invention relates to sound recording methods and systems, and particularly to the rerecording of sound whereby a sound record, either film or disc, is reproduced and again recorded as another film or disc record.

Rerecording is for several purposes, one being to combine several previously recorded records such as dialogue, sound effects, and music into a composite reco-rd for use with a motion picture.

Another purpose is to vary the level and/or fre 1 quency characteristics of certain passages or sequences of a previously recorded musical selection, especially a used record Where the noise level has increased with use to the point where it is outside of commercial limits. In the event such a record has a high noise to signal ratio, or a high surface noise level, the record can be made satisfactorily quiet in many instances by reducing the reproduced amplitude of the noise during no signal intervals in the selection by the method and system disclosed and claimed in my Patent No. 2,299,398 of October 20, 1942. However, in cases Where a considerable portion of the selection is at a low level, much of which is below the noise level, the elimination or reduction of the noise level will also eliminate or reduce the low level passages.

It has been found, however, that the surface noisev on most records lies predominately in the upper portion of the audio frequency spectrum, and thus, by selective frequency discrimination in level, the major portion of the noise may be eliminated. That is, the level of both signal and noise is reduced, but, by equalization, the high frequency portions of both are reduced to a larger degree than the low frequency portions. This action may so change the Wave form of the entire passage that it may sound fiat and dull. To restore brilliance to the altered passages, the reduced and equalized portions of both the noise and signal are passed through a reverberation chamber which accentuates and builds upthe level of the low level passages with the reverberated signal which blanketsy the noise and substantially restores the signalY to its original quality.

This method of rerecording greatly improves a-noisy record by both eliminating most of the noise during the no signal intervals, and also the noise in the passages having a level below that of the noise without eliminating the passages.

This may be accomplished either manually or semi-automatically, bothv types of operation being disclosed hereinafter.

The principal object of the invention, therefore, is to improve the recording of sound records.

Another object of the invention is to improve the rerecording of sound records.

A further object of the invention is to provide an improved system of recording or rerecording sound to eliminate noise'.

A still further object of the invention is to provide an improved method of and system for improving the vquality of a record during the rere'cording operation.

Although the novel features which are believed to be characteristic of this invention will be pointed out with particularity in the appended claims, the manner of its organization and the mode of its operation will be better understood by referring to the following description read in conjunction with the accompanying drawings forming a part hereof, in which:

Fig. 1 is a diagrammatic view of a rerecording system embodying'the invention, and

Fig. 2 is a graph illustrating the principles of operation of the invention.

Referring now to Fig. 1, a sound film reproducing system is represented by a light source 5, a lens 6, a sound film l', a lens 8, and a photoelectric cell IS. The cell IU is polarized by a battery Il over a load resistor I2, the cell output being impressed on an amplifier I5 over condenser I4 when a switch I6 is thrown to its right-hand position, and on an expander and variable equalizer unit ll when switch I6 is thrown to its lefthand position. A disc record reproducing system is represented by a disc record 20 and a pick-up Zl which is connected to switch I6 over conduc tors 22 when a switch 23 is closed.

rhe output of the amplifier l 5 is fed to a mixer or mixing panel 25 and then to an equalizer 26', the output of which is divided between an amplifier 2l, and a speaker 30 in a reverberationy chamber 32. The amplifier 21 feeds a recorder 2B, either film or disc, and a monitoring speaker 29. The reverberated output of speaker 30 is detected by a microphone 33 feeding an amplifier 35, which, in turn, feeds into the recorder 28 and monitor 9v through a volume control 31. The output of unit I1 is also connected to the speaker v'izer signal.

3 E and amplifier 21 through a mixer 3|, but the unit I1 has no output unless switch I6 is thrown to the left.

To explain the operation of the above system, reference is made to Fig. 2 which represents the signal range on a record to be rerecorded and the noise level thereon, the width of the curves representing the relative amounts of the frequencies in each component. That is, the noise curve widens toward the high frequency end of the audio spectrum to represent that most of the noise lies nin the 'high frequency end of the spectrum, while the signal, over an entire selection, contains all the frequencies to an approximately equal extent. Also, there is usually very little noise at the extreme low end of the audio spectrum.

Now assuming switch I6 is thrown to the right and a record is being reproduced having signal and noise levels, such as shown in Fig. 2. When the signal level drops to where it is lower than the noise level, the operator makes two adjustments. He will adjust his equalizer to attenuate the high frequency portion of the spectrum as `shown by the dotted portions of the low signal level and noise level curves, and he also reduces the overall level at mixer 25. Thus, the actual level fed to amplifier 21 is lower than those shown in Fig. 2. The 10W level signal and noise is also ,fed to the reverberation chamber 32 where it is reverberated, amplified at 35, and mixed with Athe equalized and reduced signal from amplifier 21. Passage through the chamber 32 emphasizes 'the signal frequencies to add brilliance thereto and also blankets the noise, since the ratio of noise to signal is much less and generally in the proper direction in the lower frequency range than in the upper frequency range, the reverberation being more effective in the lower frequency range because of the greater amplitude of .the signal. The amount of reverberated signal fed into the original signal is proportioned by the volume control 31.

When the high levels are being reproduced, the mixer increases his transmitted level, while restoring the equalizer to normal, and reduces the A.amount of reverberated signal introduced into the recording. By such a manual control, the vmixer can vary the over all level, the degree of attenuation of the high frequencies in the low signal levels and noise level, and the amount of reverberated signal being mixed with the equal- Thus, if a record varies in noise components during a selection, he may vary the amounts of volume reduction and equalization accordingly. A sound record from the disc 20 may likewise be controlled when the switch 23 `is closed.

The above action may be obtained automatically with the switch I6 thrown to its left-hand position -by the use at l1 of a quench circuit, 'shown in Fig. 1 of my Patent No. 2,224,914 of December 17, 1940, or an expander variable equalizer circuit in accordance with the yprinciples disclosed in Singer Patent No. 2,341,336 of February 8, 1944. In this Singer patent, a combination compressor and volume controlled variable equalizer is shown wherein as the volume increases, the attentuation of the high frequencies increases. By reversing the bias connections from the rectied to the variable gain amplifier and adjusting the control current amplification, the low levels of noise and signal will receive a drooping frequency characteristic, and as the Ljpredetermined minimum amplification with a.

signal level increases, the amplification will increase rapidly and then become substantially constant, the frequency characteristic becoming flat at the higher amplitudes where the normal signal level blankets the noise. My circuit in Patent No. 2,299,398, mentioned above, may also be used in association with a volume controlled variable discriminator. Thus, the level may be varied automatically with the unit I1, although the operator may initially adjust his level at 3l and his reverberated level at 31.

In the rerecording of disc records where surface noise was high and the low levels were down below that of the noise, the manually controlled system was very eiective in restoring them to their original quality and in some instances, showing an improvement over the original. In straight rerecording from normally satisfactory records, the eliminating of noise during no signal intervals as disclosed in my above mentioned Patent No. 2,299,398, and lling in with a reverberation signal enhances the quality of records so rerecorded by effectively providing a more silent record.

I claim as my invention;

1. The method of changing the signal to noise rati-o of a recorded signal, the lower levels of certain passages of which are below the noise level of the record, comprising translating recorded waves into electrical waves, attenuating the high frequency portion of low level currents corresponding to low level recorded passages of said sound waves, generating acoustic waves corresponding to a portion of the attenuated currents, reverberating said acoustic waves, generating currents corresponding to said reverberated acoustic waves, and combining a portion of the attenuated currents with currents corresponding to the reverberated acoustic waves.

2. The method of claim 1, in which the entire frequency range of said signal and noise is reduced during the transmission of low levels thereof while transmitting the high level portions of said signal without substantial modification in amplitude or frequency.

3. A sound rerecording system comprising means for reproducing a sound record, means for variably amplifying sound currents from said record, means for variably attenuating the high frequencies of the sound currents from said record, a reverberation chamber, means for generating acoustic waves corresponding to said modied currents in said chamber, means for detecting said reverberated waves, means for combining said modified currents before reverberation and currents corresponding to said reverberated waves, and means for recording said combined currents.

4. An electrical transmission system comprising a source of electrical currents corresponding to signal sound Waves, means for modifying the over-all level of said currents, means for additionally varying the level of the high frequency portions of said modified currents, means for re- (verberating sound waves corresponding to a part of the energy of said modified and varied currents, and means for combining said currents -corresponding to said modified and varied currents before reverberation with currents corresponding to said reverberated sound waves.

5. A transmission system in accordance with claim 4, in which currents corresponding to noise in said source of electrical currents are simultaneously modied by said system.

6. A sound recording system comprising a source of electrical currents corresponding to an audio signal and noise, means for modifying said signal and noise by attenuating the level of the frequency range in which said noise is present, means for reverberating acoustic waves corresponding to a part of the energy of said modied currents, and means for combining currents corresponding to said modied currents before reverberation with currents corresponding to said reverberated waves.

7. A recording system in accordance with claim 6, in which means are provided for reducing the over-all level of said first mentioned currents during the transmission thereof.

BARTON KREUZER.

REFERENCES CITED The following references are of record in the ile of this patent:

UNITED STATES PATENTS 

